How aluminum oxide's liquid-like properties protect nuclear plants

MIT researchers published a paper this week where they explain that thin layers of solid oxide protective coating for metals can deform as if they were liquid, filling any cracks and gaps as they form.

In a press release, the scientists explain that the coating layer should be especially useful to prevent the leakage of tiny molecules such as the hydrogen gas used to power fuel-cell cars or tritium, a heavy form of hydrogen that forms inside the cores of nuclear power plants.

In detail, the MIT team led by graduate student Yang Yang used an environmental transmission electron microscope to observe the surface of metals coated with aluminum oxide, chromium oxide, and silicon dioxide, which produce an oxide that can actually serve as a protective barrier to prevent any further oxidation.

The device allowed them to work in the presence of gases and liquids of interest (such as water and oxygen) and observe the process that can lead to a type of failure known as stress corrosion cracking.

What they noticed, however, was that aluminum oxide has just a liquid-like flowing behavior, even at room temperature, if it is made into a thin enough layer, about 2 to 3 nanometers (billionths of a meter) thick.

Thanks to this quality, the usual corrosion process does not take place. "Even with a solid protective layer, cracks can form that allow the oxygen to penetrate to the bare metal surface, where it can then penetrate into interfaces between the metal grains that make up a bulk metal material, causing further corrosion that can penetrate deeper and lead to structural failure," the brief explains.

But based on the researchers' observations, it is understood now that an aluminum oxide layer, normally so brittle it would shatter under stress, when made exceedingly thin "is almost as deformable as a comparably thin layer of aluminum metal — a layer much thinner than aluminum foil. When the aluminum oxide is coated onto a surface of a bulk piece of aluminum, the liquid-like flow keeps the aluminum covered with its protective layer."

The experiment also demonstrated that the aluminum with its oxide coating could be stretched to more than double its length without causing any cracks to open up.